F. Zocca

An Advanced Preamplifier for Highly Segmented Germanium Detectors

We present a fast low-noise hybrid charge-sensitive preamplifier for germanium position-sensitive gamma-ray detectors. In conjunction with a bulky 36-fold segmented detector it provided an excellent resolution of 1.71/1.77 keV fwhm on the 1.17/1.33MeV60Co lines. The preamplifier rise time, as measured at the test bench, is as fast as 7.5 ns, with a detector capacitance of 21 pF and with a 5 m 50 Omega twisted-pair cable connected at its output. The dynamic range of the preamplifier input stage is as large as 92 dB, ranging from 0.275 fC to 9.9 pC, i.e., from 5 keV to 180 MeV in terms of photon energy. On signals larger than 2 MeV a fast reset (~ 10 MeV/mus) is enforced to reduce the system dead time. An estimate of the amplitude of such large signals is derived from the reset time, still obtaining a high resolution. Using this technique we achieved an energy resolution of 0.3% at 16.7 MeV

A Time-Over-Threshold Technique for Wide Dynamic Range Gamma-Ray Spectroscopy With the AGATA Detector

Wide-range energy measurements have been carried out with an encapsulated 36-fold segmented germanium detector of AGATA (Advanced GAmma-ray Tracking Array). An unprecedented equivalent energy range of 5 keV to 180 MeV, or ~90 dB, has been obtained using a single acquisition chain, while maintaining a high energy resolution in the whole spectrum. For energies larger than 3 MeV the pulse-height is obtained with a Time- over-Threshold (ToT) technique. The measured resolution in all the tested range from 3 to 50 MeV is of <0.4%. The ToT technique has been demonstrated using a 241Am + Be source with Ni target generating high-energy gamma photons by neutron capture reactions. A remarkable resolution of 0.21% has been obtained on the Ni spectrum line at the energy of 8.998 MeV.

Cryogenic Performance of a Low-Noise JFET-CMOS Preamplifier for HPGe Detectors

Cryogenic low-noise charge sensitive preamplifiers have been developed and realized for the GERmanium Detector Array (GERDA). An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ~60 pF. An optimum resolution of 1.6 keV FWHM has been obtained for the pulser line at 6 ¿s shaping time. A resolution of 2.1 keV FWHM has been achieved for the 1.332 MeV line from a 60Co source. A wide bandwidth (rise time of ~16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the crystal. A low power consumption (~23 mW) makes the preamplifier suitable for a multi-channel array of germanium detectors.

Extending the dynamic range of nuclear pulse spectrometers

Using an innovative time-varying front-end electronics in conjunction with a bulky coaxial high-purity germanium detector, we were able to extend the range of the radionuclide spectra well beyond the analog to digital converter (ADC) saturation point. The electronics automatically conditions the signal for digital-filtering optimization if it is in the ADC voltage range and for time-over-threshold analysis if it exceeds the ADC range. A high spectroscopic resolution has been achieved in both operation ranges. An unprecedented wide energy range from 5 keV to 150 MeV of equivalent energy, or 90 dB, has been obtained using a single acquisition chain, while maintaining a high-energy resolution in the whole spectrum. For example, with an ADC range of 3 MeV a resolution has been obtained of 1.3/2.2 keV full width at half maximum on the 122/1332 keV gamma-ray lines of 57Co and 60Co, and of <0.4% in the time-over-threshold region, or for energy deposits beyond 3 MeV.

Design and Optimization of Low-Noise Wide-Bandwidth Charge Preamplifiers for High Purity Germanium Detectors

Design criteria for low-noise wide-bandwidth charge-sensitive preamplifiers for highly-segmented HPGe detectors are presented. The attention is focused on the optimization of the preamplifier noise, long-term gain stability, and bandwidth. The charge-sensitive preamplifiers of AGATA, i.e., the Advanced GAmma Tracking Array detector for next generation nuclear physics experiments, have been designed, realized and optimized using the proposed techniques. The circuit, in conjunction with the detector, provided an Equivalent Noise Charge of 101 electrons r.m.s., a rise time of ~ 8.3 ns, no appreciable line shift in long term acquisitions, a dynamic range of as much as 92 dB. An analytical study of the circuit is made. Computer simulations and experimental results are shown and critically discussed.

A JFET-CMOS Fast Preamplifier for Segmented Germanium Detectors

A JFET-CMOS fast charge-sensitive preamplifier for germanium detectors, able to operate at cryogenic temperatures, has been designed, realized, and characterized. The monolithic part of the circuit is realized in a mature 5 V 0.8 m Si CMOS technology, which yields better performances than scaled technologies in this case. The input transistor is an external Si JFET, which can be easily replaced if necessary. The charge-to-voltage gain and the fall-time are as well set through an external RC network. The circuit works in the wide temperature range of 196 to 55 C and is able to drive a terminated coaxial cable with an exceptionally fast and clean transition. Namely, with a detector capacitance of 16 pF and a negative power supply of 3 V it is able to provide a 2.4 V pulse onto a 100 load in less than 13 ns with no ringing. The static power consumption is 8 mW excluding the JFET. The area occupancy of the integrated circuit is as little as 366275 . The noise performance with a 16 pF detector capacitance is 110 r.m.s. electrons both at room temperature and at C, at a quasi-Gaussian shaping time of 10 s. The obtained performance is adequate for gamma-ray spectroscopy and pulse-shape analysis with bulky HPGe segmented detectors.

Low-noise amplification of /spl gamma/-ray detector signals in hostile environments

We designed and realized a low-noise charge preamplifier for HPGe (High Purity Germanium) /spl gamma/-ray detectors, able to operate at a distance of 3 m to 6 m from the detector. One transistor only is placed in close proximity to the detector. Such a setup is required in applications where the detector works in hostile environments that could damage or destroy the electronic circuitry. Using 3 m RG62 cables and a 23 pF detector capacitance we obtained a noise of /spl sim/1.07 keV fwhm at 2 /spl mu/s shaping time, so fully compatible with /spl gamma/-spectroscopy requirements. By compensating the preamplifier so as to completely eliminate the ringing in its response function we obtained a rise-time of /spl sim/46 ns with 3 m cables and of /spl sim/80 ns with 5 m cables. With a different approach, or using a lower compensation capacitance and eliminating the ringing through a numerical post filter, we obtained a faster rise time of /spl sim/33 ns, with a detector-preamplifier distance of 5 m, while maintaining the low-noise performance. This latter setup is adequate for spectroscopy and tracking of /spl gamma/-rays with segmented HPGe detectors.

An integrated low-noise charge-sensitive preamplifier with virtually unlimited spectroscopic dynamic range

A low-noise ASIC preamplifier for semiconductor detectors has been built and characterized, which is able to provide a linear spectroscopic measurement of the detector charge signals even when its output voltage is saturated. The preamplifier works in a conventional mode, i.e. as an active current integrator, when the output signal is below a preset threshold. If the signal is larger the preamplifier operates in a non-standard controlled fast-reset mode, which is effective in providing a linear measurement of the detector signal charge even when the preamplifier works close to or in saturation. The experimental relation between the measurement and the input charge keeps perfectly linear irrespective of the non-linearity of the preamplifier working regime, which yields a dynamic-range boost of more than one order of magnitude.

Cross-Talk Limits of Highly Segmented Semiconductor Detectors

Cross-talk limits of monolithic highly-segmented semiconductor detectors for high-resolution X-gamma spectrometry are investigated. Cross-talk causes false signal components yielding amplitude losses and fold-dependent shifts of the spectral lines, which partially spoil the spectroscopic performance of the detector. Two complementary electrical models are developed, which describe quantitatively the interchannel cross-talk of monolithic segmented detectors whose electrodes are read out by charge-sensitive preamplifiers. The first is here designated as Cross-Capacitance (CC) model, the second as Split-Charge (SC) model. The CC model builds around the parasitic capacitances Cij linking the preamplifier outputs and the neighbor channel inputs. The SC model builds around the finite-value of the decoupling capacitance CC used to read out the high-voltage detector electrode. The key parameters of the models are individuated and ideas are shown to minimize their impact. Using a quasi-coaxial germanium segmented detector it is found that the SC cross-talk becomes negligible for decoupling capacitances larger than 1 nF, where instead the CC cross-talk tends to dominate. The residual cross-talk may be reduced by minimization of stray capacitances Cij, through a careful design of the layout of the Printed Circuit Board (PCB) where the input transistors are mounted. Cij can be made as low as ~5 fF, but it is shown that even in such case the impact of the CC cross-talk on the detector performance is not negligible. Finally, an algorithm for cross-talk correction is presented and elaborated.

A cryogenic low-noise JFET-CMOS preamplifier for the HPGe detectors of GERDA

Cryogenic low-noise charge sensitive preamplifiers have been realized and tested for the GERmanium Detector Array (GERDA). In the search of neutrino-less double-beta decay of 76Ge at LNGS, GERDA will operate bare segmented germanium detectors immersed in liquid argon. The front-end electronics will operate in the cryogenic liquid too. An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been developed and realized. It has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ∼ 60 pF. An optimum resolution of 1.6 keV fwhm was obtained on the pulser line at 6 μs shaping time. The obtained resolution for the 1.332 MeV line from a 60Co source was of 2.2 keV fwhm. No peak shifts or line broadenings were seen during long-term acquisitions, thanks also to the extremely high preamplifier loop gain which yields a very high closed-loop gain stability. A wide bandwidth (rise time of 16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the detector. A low power consumption (23.4 mW) makes the preamplifier suitable for the foreseen multi-channel array of germanium detectors.